What language is the default for this platform?

I dunno, whats your instance configured like? Mine defaults to German.

What language is used for the TOS?

I dunno either, whats your instance configured like? The TOS of my platform is written in German.

What language do the developers use?

A mix of Rust, TypeScript, and PLpgSQL; and also French as Dessalines is a frenchman, but for global communication I guess the team uses English (which is still an official language in the EU)

The Fediverse is a global project, but it’s also mostly English based.

Just because you overwhelmingly see English content doesn’t mean that its mostly like that.